Mobile radio is today presented with a number of alternative and largely incompatible signal transmission systems, including the analog system of AMPS, the North American Digital Mobile Radio Standard IS-54, and Qualcomm's CDMA system. The systems each require dedicated, application-specific hardware and most of the hardware designed for one system is useless with the others. Furthermore, although various techniques have been proposed in each of these systems to improve performance and increase capacity by reducing signal interference and fading problems, implementation of these techniques in any given system is often seriously constrained by the expense entailed in the required hardware addition and/or replacement.
For example, the use of space diversity reception at the base station has been proposed to overcome fading and signal interference. In a space diversity radio system, signals received by a plurality of antennas are processed to overcome fading and interference due to other users. See, in this regard, "Optimum Combining in Digital Mobile Radio with Cochannel Interference", J. H. Winters, in the IEEE Journal on Selected Areas in Communications, vol. SAC-2, No. 4, July 1984. Such processing can include multiplying the signal from each antenna element by a controllable weight to adjust the phase and amplitude of the signal and then combining these signals to generate the output signal. The pattern of the adaptive array is thereby changed to reduce the power of interfering signals and to optimize desired signal reception, substantially increasing the capacity and performance of mobile radio systems. Similarly, multiple transmit antennas at the base station can be used to improve the capacity and performance at the mobile. In particular, using optimum combining, whereby the signals are weighted and combined to minimize the mean square error in the output signal, an M-fold increase in capacity can be achieved with M antennas, as shown in "The capacity of wireless communication systems can be substantially increased by the use of antenna diversity", R. D. Gitlin, J. Salz, J. H. Winters, in the Proc. of the First International Conference on Universal Personal Communications, Sep. 29-Oct. 2, 1992, Dallas, Tex., pp. 28-32. Thus, modular growth can be achieved by simply adding antennas to the base station. Although the major cost of such improvements is now at the base station, where the cost is shared among the users, the magnitude of the investment required to add to and modify the hardware to handle the additional antenna signals at the base station can be staggering, particularly when considering the large number of channels the base station may have to process, e.g., up to 333 channels/users in the AMPS system or 1000 users with 333 frequency channels in an IS-54 system. Such investment is even less likely if the hardware must be replaced with each system change.